Replacing the _CACHE_MAP_ file with an older copy is not a good idea.
I'm not sure if the Cache fixer extension still works with current Firefox versions, but you can give the version posted here a try:
*http://forums.mozillazine.org/viewtopic.php?f=38&p=8561315#p8561315
The maxVersion is 4.0, so you may need to bump it by editing the install.rdf file.
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*http://imglikeopera.mozdev.org/cache_fixer.html